Type narrowing in loose equality fails for edge cases like empty string and zero

TypeScript Version: 3.8.3

Search Terms:
type narrowing loose equality
type guard loose equality
loose comparison empty string zero
non-strict comparison empty string zero
type guard empty string zero
coercion double equals
type guard coercion fails

Code

TS misses edge cases when narrowing down types in loose equality comparisons.

Following the example from the new handbook on Equality narrowing, if you replace the strict equality with a loose equality, you're rocket is ready to explode. If you pass in the empty string and zero the if condition passes and accessing toUpperCase() on a number fails.

function foo(x: string | number, y: string | boolean) {
    if (x == y) {
        // We can now call any 'string' method on 'x' or 'y'.
        x.toUpperCase();
        y.toLowerCase();
    }
    else {
        console.log(x);
        console.log(y);
    }
}

foo(0, "");

It might be worth mentioning, that this is only one out of many edge cases with the loose equality operator. Maybe you want to check more than just my example above. I bet Kyle Simpson (@getify) could give you some more examples.

Expected behavior:

After a loose equality comparison, TS should not narrow down string | number and string | boolean to string, since a number can equal a string in the edge case 0 == "".

Actual behavior:

After a loose equality comparison, TS narrows down string | number and string | boolean to string, even though this is incorrect, as seen for edge cases like 0 == "".

Playground Link: Playground Link

Related Issues:

TypeScript 3.8.3 を使用したリンク先の Playground reproduction から始め、0 == "" に対する JavaScript の loose equality の挙動と narrowing の結果を比較してください。equality-narrowing の挙動と関連するエッジケースを確認してください。runtime coercion によって比較が true になり得る場合に、checker がその例を安全に narrowed されたものとして扱わなくなれば完了です。